Research this past year focused on the molecular characterization of glutamate receptors and their function in the auditory system. Three families of inotropic glutamate receptors are now known: AMPA, kainate and NMDA receptors. In addition, two related subunits, delta 1 and 2, have been identified but their functions remain unknown since they are not functional ion channels when expressed in oocytes or cultured cells. Our efforts to develop subunit-specific antibodies to these proteins to study their distributions and biochemical properties continued as antibodies selective for NMDAR2A/B and delta 1/2 were made. Immunoprecipitation studies show that the delta subunits are not associated with AMPA, kainate or NMDA receptors; and that the delta receptor in brain does not bind the ligands, AMPA, kainate or glutamate, which bind with high affinity to other glutamate receptors. While the delta subunits were distributed throughout the brain, they were most concentrated in cerebellar Purkinje cells; at the ultrastructural level, immunoreactivity was intense postsynaptic of parallel fiber input. NMDAR2A/B is widely distributed throughout the brain, with a pattern similar to that of NMDAR1. At the ultrastructural level, antibodies to NMDAR2A/B stained postsynaptic densities. Antibodies to the NMDAR1 splice variants and NMDAR2 subunits are being used to study the NMDA receptor structure and association with the cytoskeleton. In situ hybridization histochemistry, PCR amplification and immunocytochemistry were used to study glutamate receptor expression in the cochlea. Our results suggest that AMPA, kainate and NMDA receptors are all functional at the hair cell/spiral ganglion cell synapse. In the cochlear nucleus, we are working to identify receptor subunits expressed and neurons in which they are expressed. In situ hybridization showed kainate receptor subunits are only lightly expressed in the cochlear nucleus with only fusiform cells have complements of subunits likely to produce functional receptors. AMPA, kainate and NMDA receptors appear to have distinct, but overlapping patterns of expression. mGluR1a and the delta receptor are relatively abundant in the dorsal cochlear nucleus.